How to find Whirlpool and Pinwheel Galaxy?

[Stu]

Hi,

All your eyepieces magnify to a varying extent depending on the focal length. You can calculate the magnification by dividing the focal length of the scope (1250mm I believe), by the focal length of the eyepiece.

Your 4, 9, 20 and 30 mm give magnifications of x312, x139, x63 and x42 respectively.

The 4mm will be too much except perhaps on the moon.

The 9mm will be useful on planets, globular clusters and things like M57 which need higher powers. The 20mm is good for larger objects such as open clusters and the 30mm as a finder eyepiece or for larger objects.

One other thing to get familiar with is exit pupil. This is calculated by dividing the aperture by the magnification, or alternatively dividing the focal length of the eyepiece by the focal ratio of the scope. This figure is in mm and represents the diameter of the beam of light which exits the eyepiece into your eye, and it determines the brightness of the image.

At the larger end, there is little point going larger than the maximum size of your fully dilated pupil as this wastes light, gives washed out images if you have light pollution, and in extreme cases the shadow of the secondary mirror can start to be visible.

At the lower end, exit pupils below 1mm can cause the image to be dim, and can also cause floaters to become visible when observing brighter objects such as planets.

So, the 30mm would give an exit pupil of:

254mm/42mm or

30mm/4.92 = 6.09mm

This is a good low maximum as a finder eyepiece, or for viewing larger nebulae or open clusters. It may seem washed out if your skies aren't dark enough.

The 20mm gives 4mm, the 9 gives 1.8mm and the 4 gives 0.8mm. Exit pupils around 2mm give good contrast for observing smaller DSO's so as said before the 9mm will be a handy eyepiece.

The 4mm will be quite dim and a little blurry on anything other than the moon.

Last concept is field of view. This is calculated by dividing the apparent field of view of the eyepiece, by the magnification it gives.

I'll assume your eyepieces are plossls with a 50 degree apparent field of view, but you can check this.

So, the eyepieces give the following fov's

30mm gives 1.19 degrees

20mm gives 0.8 degrees

9mm gives 0.36 degrees

4mm gives 0.16 degrees

As an example of these, here are some charts with those fields of view represented as circles, overlaid on M57 and showing you what to look for.

The first ones are presented correct way around:

Once you've found Vega, M57 is between Sheliak and Sulafat.

This last one is shown upside down as you would see in the Newtonian. Positioning Sheliak on the edge of the field should bring M57 into the view. The exact orientation may be a little different but you hopefully get the idea.

You may want to consider something around the x180 mark for planetary use, something around 7mm may be good.

Hope that's of some use.

Cheers,

Stu

[YKSE]

Excellent explanation Stu.

Hi, On my 150pds with up to 8mm eyepiece, I cannot visually see either of these Galaxies. And I know I am in the right position because I try to observe immediately after imaging them. I think they are very faint, especially in light polluted areas. Might be different in dark skies. Tim

If you're looking for M101 with 8mm eyepiece, I doubt you'll ever be able to see it, even in a dark site. M101 is relatively big and has relatively low surface brightness, it'll easier to see with eyepieces with lower magnification (for larger exit pupils), as explained above by Stu.

[Future_AstrophysicsGirl17]

Great, great explanations Stu, as YKSE also said above. I'm beginning to learn how to really use my lens and their technical aspects, so a big thank you awarded to that. It makes much more sense now 

[jetstream]

Eyepiece field of view can make a difference at times on visually large, faint objects, as you can end up "looking through it" pretty easy. The lower mag eyepieces compensate well for this but can make for hard finding with less than dark skies.

M101 is strange in a sense...it will show spirals under very dark skies but can be almost invisible with any light pollution. M101 is not easy from urban or near urban areas IMHO.

The view under dark skies can be amazing though...

I use 24 Canum Venaticorum to find M51, as is shown in the above simulations. If you can't see this star @ 4.6 mag, its best to find another object- Cassiopeia is a good alternative hunting ground.

[Future_AstrophysicsGirl17]

Hello all, I am in a fantastic mood tonight! I finally saw the Ring Nebula 20 minutes ago! It didn't take that long, and following the advice you guys gave me, I was able to see the fuzzy, gray ring with my 30, 20, and 9mm lenses. Some day I should invest in a DSLR camera, because a simple phone camera just couldn't do it, of course. I admit that I almost cried instantly...what a huge sense of accomplishment

Still no luck with M101, it's a strange galaxy. I think I got close to seeing M51 tonight but I need slightly darker skies. Having an aligned finder scope really does help. I will post again once I have success with one of these two. Good night, everyone

Chloe  

[Davey-T]

Well done Chloe, nice for an old cynical astronomer to hear the successes of a young enthusiastic one 

Dave

[Davesellars]

Concentrate on M51 first before M101.  In fact, forget about M101 at the moment until you've got more experience seeing other galaxies that are brighter.  M51 is actually one of the brighter galaxies but still needs a reasonably dark sky (NELM of 4.5 I would say is the limit) with good transparency to be appreciated.  Start with something like a 20mm EP to search it out then increase the magnification.

I can see approx stars of magnitude 5 directly naked eye from my observing site on a good dark night with no Moon and M101 is just about visible with averted vision, fully dark adapted and all other trickery with my 8" SCT.